Descriptive Statistics

** Introduction to R-Studio: Descriptive Statistics**

Note

Note: Today we will go over how to create a data set to work with using the rnorm and pnorm functions. We will use the functions mean, median, and range to get measures of central tendency.

Data Generation

Tip

When we do not have data to work with, we can create it ourselves. It is important to note that this is only useful, and ethical, when we are doing it for instructional purposes!

The function rnorm has a few arguments that help specify the type of data that it will return: - rnorm(n,mean,sd) - n = number of observations - mean = desired mean of the sample - sd = desired standard deviation of the sample






In the above case, it was specified that we wanted R to return us a dataset that had an n of 10, a mean of 10, and a standard deviation of 1.


There are some instances of preference when it comes to data generation,but I prefer the data to look a bit cleaner so I will usually:


    

  • round the data so that there are two decimal places after the 0
    

      

    • We can do this by wrapping the round function around our rnorm function. round has one argument, digits=n, where n is the number of digits you would like to round to. In practice it should look like this:
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  • exclude numbers below 0 from the data.
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Try it out: Make a variable called y and make sure that it has 30 observations, a mean of 5, and a standard deviation of 5. Make sure that the data is rounded to 0 decimal places and does not include any value greater than 100 or less than 0.






  





We can now assume that data we have generated could have come from a class that just recently took a test. What we would like to do next is find out some of the generalities of this data.






Descriptive Statistics


    

  • What is the average score?
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  • What score is right in the middle?
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  • What is the range of scores?
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We can do this using the mean, median, and range functions.


All of these functions work by placing the object or variable you are looking to get descriptive data from.


When you are dealing with ‘real data’ you should make sure to give the dataset a descriptive name so you understand what you are working with. Let us rename the variable y to be called testscores instead.






  





Now that we have workable data we can start to look its properties.






  









  









  





We can see that we have a mean of 79.12, a median of 79, and a range of 17.


In addition to using these three functions to extract summary data, we can use another function aptly named summary. This function returns the above three values as well as two additional values referencing datapoints in the 1st and 3rd quadrants.


The summary function works by placing the variable you want to summarize inside the parentheses.






  





It might also be helpful for us to see what the standard deviation is for this dataset. We can do this by using the sd function.






  





We now have the ability to tell this professor that the average grade of her students was 79.12, and the standard deviation was 4.94402.


This should not be too surprising to us seeing as how we designated the properties of this data set when we used our first rnormfunction!












Note







Try it out: Get the summary data for the following data: